1. Field of the Invention
The present invention relates to a silver alloy target for forming a reflective electrode film for an organic EL element, and a method for manufacturing the silver alloy target. More specifically, the present invention relates to a large-sized silver alloy target for forming a reflective electrode film for a organic EL element.
2. Description of the Related Art
Conventionally, as display devices, the cathode-ray tube (CRT), the liquid crystal display (LCD), the plasma display (PDP), the light emitting diode (LED), the EL display, or the like have been known. Such display devices are widely used as the back panel of the computer display and the liquid crystal display.
Among these, organic EL elements are recently receiving considerable attention and are light-emitting elements using the principle in which a voltage is applied between a positive electrode and a negative electrode formed on either side of an organic EL light-emitting layer, positive holes and electrons are injected into an organic EL film from the positive electrode and the negative electrode, respectively, to thereby emit light from the organic EL film when positive holes and electrons are combined in the organic EL light-emitting layer.
An organic EL element driving method includes the passive matrix type and the active matrix type. The active matrix type can provide high-speed switching for each of pixels by providing one or more thin-film transistors, and thus, is advantageous for high contrast ratio and fineness and can exhibit the characteristics of organic EL elements.
The light extraction method includes the bottom emission type that extracts light from a transparent substrate side and the top emission type that extracts light at the opposite side of the substrate. The top emission type which has a high aperture ratio is advantageous for obtaining a higher luminance and may become a future trend.
FIG. 1 shows an example of the configuration of a layer in a top emission structure where the reflective electrode is a positive electrode. Here, in order to efficiently reflect light emitted from an organic EL layer, it is preferable that a reflective electrode film (described as a “reflective positive electrode film” in FIG. 1) provide high reflectivity and high corrosion resistance. It is also preferable that the reflective electrode film has a low resistance as an electrode. Such materials known include Ag alloy and Al alloy. In order to obtain an organic EL element having a higher luminance, Ag alloy is excellent in high visible light reflection.
Here, a sputtering method is employed for forming a reflective electrode film for an organic EL element, and a silver alloy target is employed (Patent Document 1).
With increasing the size of a glass substrate during the production of an organic EL element, a large-sized silver alloy target used for forming a reflective electrode film has also come to be employed. Here, when a large-sized target is subjected to sputtering by applying high electric power thereto, a phenomenon called “splashing” generated by abnormal discharge in the target occurs. Consequently, molten micro particles may be attached to the substrate to thereby cause a short circuit between wirings and electrodes, resulting in a decrease in the yield of the organic EL elements. In a reflective electrode layer of the top emission type organic EL element, a higher flatness is required in order to attain an undercoat layer for an organic light-emitting layer, and the occurrence of splashing needs to be suppressed.